Methods of vacuum deposition in a glow discharge can be classified in two groups. The sputtering method, by far the best known and oldest one, consists in detaching the atoms of the material forming a cathodically connected target by bombarding it with ions of the gas present, and then condensing the atoms on a substrate. The other method consists in evaporating the material of the anodically connected target, and depositing the evaporated particles on a substrate connected to form a cathode at high negative potential and thereby subjected, simultaneously with the deposition of the evaporated substance, to intense bombardment by ions of the inert gas present in the glow discharge. Because of this bombardment, which cleans the substrate, the source has to produce more particles than the cleaning removes. This second method has great advantages over the first. The cleaning resulting from the ionic bombardment of the substrate and the projection of the particles onto the substrate give the deposit a much better adherence than if the deposit is produced by sputtering. As the material of the target can be evaporated at a high rate by means such as an electron gun, the deposition rate is greater than that obtained by sputtering in spite of the cleaning undergone by the substrate. However, this latter advantage limits the use of the method in certain circumstances. Thus, it is for example difficult to transmit the evaporated material from above downwardly or to make a deposit on a large surface. Consequently, great difficulties are encountered when it is required to use this deposition method on loose pieces, because if the pieces are placed in a basket situated below the target, most of the material will be deposited on the basket and not on the pieces which it contains.
It has been proposed to overcome this disadvantage by placing above the target deflector screens heated to a temperature higher than the condensation temperature of the evaporated material, so as to direct the vapor downwards. This method lowers the efficiency and considerably increases energy consumption. Moreover, it is impossible to use such screens with very reactive metals or with metals having a low vapor pressure.
Sputtering has also been used for deposition on a weakly polarized conducting substrate. As the principle underlying the sputtering of the target is the same as that underlying the cleaning of the substrate, the cleaning of the substrate must of necessity be very slow if a deposit is to be obtained. To this end, the substrate is polarized to a few dozen volts, 200 volts at most, so that the ionic bombardment is of low efficiency and adherence is not comparable with that obtained by intensive bombardment of the substrate pursuant to the above-described method of deposition in a glow discharge accompanied by ionic cleaning of the substrate. Moreover, the deposition rate is very low, especially in the case of deposition on items in bulk.
It has been known for some time that the presence of a magnetic field makes the glow discharge much more intense. This phenomenon, known as the Penning discharge, is used in the magnetron to compel the secondary electrons emitted by a cathode to traverse a spiral trajectory inside the magnetic field, thereby increasing the probability of ionizing collisions occurring with the atoms of the gas present.
The intensification of the discharge notably results in acceleration of the sputtering of the cathode, which is particularly advantageous for depositing coatings.
The use of cylindrical geometry for the magnetron has the disadvantage of requiring the substrate to be placed inside the cylinder. This limitation is overcome by opening the magnetron and placing a permanent magnet in the form of a closed loop behind a flat cathode, to form what might be called a flat magnetron.
The use of the flat magnetron has already been suggested in a French patent application, No. 75-33158 filed 30 Oct. 1975, published 27 May 1977 under U.S. Pat. No. 2,329,763. This French application describes a method of deposition in a glow discharge accompanied by ionic cleaning of the substrate; this technique is generally known as ion plating. However, in such a system, the coexistence of two discharges, one around the substrate and the other highly localized close to the surface of the flat magnetron, necessitates certain precautions. According to the French patent application, grids are placed between the two cathodes to reduce or suppress the interaction of these two discharges, but practice has shown that these grids become rapidly loaded with the sputtered metal, thereby progressively reducing the efficiency of the process and, consequently, diminishing one of its important advantages.